Method of treating wells



Patented Mar. 23, 1954 METHOD OF TREATING WELLS Frank H. Braunlich, Jr.,and Paul H. Cardwell, Tulsa, Okla., assignors to The Dow ChemicalCompany, Midland, Mich, a corporation of Delaware No Drawing.Application July 2, 1952, Serial No. 296,952

11 Claims.

I The invention relates to methods of treating wells formed in theearth. It mor particularly .concerns an improved method of treating oiland gas wells so as to facilitate their production.

It has long been known that the flow of fiuid, such as oil and gas, to awell drilled into a producing stratum of the earth may be facilitated byproviding either additional flow passageways in the stratum or enlargingthe existing ones by which the produced fluid reaches the well.

One of the most widely used methods of facilitating the flow of a fluidfrom a producing stratum of a well'comprises the injection through thewell into the stratum of a suitable solvent which tends to form new flowchannels to the well hole. The solvent most generally used is an aqueoussolution containing hydrochloric acid.

In the use of such solvents in the treatment of wells, it is generallyunderstood that to get the best results from the solvent it should beinjected deeply into the stratum from which production is sought. Yet,injection into the earth is not always easy and special pumps providingconsiderable pressure are oftentimes used to force the treating solutionto enter the earth formation. In fact pressures as high as thoserequired to lift the overburden of earth and to break and crack theearth layers oftentimes are used. Operations of this kind are describedin an article by Grebe and Stoesser, published in the August 1935 issueof World Petroleum, at page 481, in which it is disclosed that pumpswere developed capable of producing suflicient pressure to force theacid into newly made cracks, while the earth is compressed and evenlifted. Wells are often completely dry, but by application of acid underhigh pressure pores are developed which lead to reservoirs and moreporous portions of the formation.

Although well treating pressures high enough to fracture and lift theearth formation are oftentimes used in convetnional treatments, earthformations are encountered which take the treating solution too easilyto permit high treating pressures to be reached in the bore hole withoutxtreme rates of injection. As a consequence, cracking or fracturing ofthe formation to be treated and the lifting of the overburden by thetreating fluid is not assured in such cases without high rates ofinjection. It is apparent that when the treating fluid enters the earthreadily or under a relatively low pressure the earth formations arealready relatively pervious to fluid at the zones of treatment and butlittle, if any, beneficial effect on production can :be expected beyondthat resulting from mere pore enlargement or cleaning ofexisting flowchannels. It is the usual practice to attempt to control the penetrationof the solvent into the earth so that the relatively fluid impermeableportions are treated rather than those which are already quite fluidpermeable. In order to accomplish this relatively high treatingpressures are required and to prevent excessive penetration of thetreating agent into the more permeable zones While high pressure isexerted on the treating agent, a pore-plugging agent is used with thetreating agent. The pore-plugging agent soused flows into and blocks thepores of the more pervious zones into which the treating agent tends atfirst to flow the most readily. Thereafter, as the more permeableportions become more or less sealed by the pore-plugging agent, thetreating pressure may be desirably raised, thereby forcing the treatingagent to find ingress into the tighter and less permeable portions ofthe formation exposed in the well hole to the treating agent, as bybreaking down or fracturing the formation.

In applying the foregoing procedure, certain difficulties arise whichhave not heretofore been adequately overcome. One of these difficultiesis that the sealing agent tends to remain in the pores of the formation,permanently plugging them after the treating pressure is released,unless the plugging material used is dislodged by earth fluids seekingtheir way to the well. To ensure pore-plugging during treatment followedby dislodgement when the treatment pressure is released, it has beenproposed heretofore to use as the pore-plugging agent an organicjellifying material mixed with suitable bacteria which cause thejellifying material to lose its pore-plugging properties throughliquefaction after a sufficient period of time has elapsed. Although agenerally satisfactory temporary pore-plugging action is therebyobtained in many wells, conditions oftentimes are encountered underwhich an adequate temporary pore-plugging action cannot be ob-' tained.The conditions involve either relatively high earth temperatures whichinterfere with the bacterial action, or excessive porosity which permitsthe jellifying material to be squeezed out of the well hole too easilyinto the porous parts of the formation before sufiicient pressure can beexerted on the treating agent to force it into the less permeableportions, the treatment of which it is desired to ensure. Insofar as weare aware, there is no commercially available pore-plugging materialwhich can be used effectively at extreme pressures and temperatures indeep wells. Accordingly, it is the principal object of the invention toprovide an improved pore-plugging agent which overcomes thedisadvantages of those heretofore available. Other objects andadvantages will become apparent as the description of the inventionproceeds.

We have discovered that by reacting together in aqueous solution awater-soluble magnesium salt and a water-soluble fluoride in thepresence of strong mineral acid a jelly-like viscous mass is obtainedwhich may be used as a pore-plugging agent that is free from theforegoing disadvantages. In addition, it possesses the property oflosing its jelly-like viscous consistency by contact with the usualearth formations for a suiflcient length of time. Hence, the pluggingaction is but temporary. The jelly-like material also exhibits theproperty of thixotropy and thus injection into the well is facilitated.The material also has the ability to suspend particulated solids, e. g.sand particles, ceramic particles, metal particles and the like, and itis, therefore, possible to include such agents in the material in theevent that it is desirable to deposit a fluid permeable mass of solidparticles in the cracks or fractures which may be formed on applyinginjection pressures sufliciently great to crack and lift the overburdenof earth. The invention then consists of the improved pore-pluggingagent and method of treating wells therewith herein fully described andparticularly pointed out in the claims.

In carrying out the invention, a soluble magnesium salt is employedwhich, in solution in Water, furnishes magnesium ions in sufflcientconcentration. Examples of suitable magnesium salts are: magnesiumchloride, magnesium bromide, magnesium acetate, magnesium nitrate,magnesium sulfate. A water-soluble fluoride capable of reacting inaqueous solution with the magnesium salt in solution, to produce amagnesium fluoride gel, is used. Examples of suitable fluorides are thealkali metal and ammonium fluorides and bifluorides.

The number of equivalent weights of soluble fluoride used should be fromabout 95 to 105 per cent of the number of equivalent weights of themagnesium salt used or preferably a mole of the fluoride salt for eachmole of the magnesium salt. The formation of the desired magnesiumfluoride gel takes place in water solution in the presence of mineralacid, in accordance with the relation: Mg+++2F-+acid MgFz+acid. Thenumber of equivalent weights of acid used is from about 45 to 350 percent of the number of equivalent weights of magnesium salt involved. Thenecessary acid may be provided by the addition of a mineral acid, suchas hydrochloric acid, to either the solution containing the magnesiumsalt or the solution containing the fluoride salt or apportioned betweenboth solutions. Alternatively, the desired acid is produced during thegel-forming reaction, without the need for supplying another acidconstituent, when bifluorides are used as the source of fluoride ion.For example, the use of ammonium bifluoride is illustrative of thisaction and is expressed in the following equation:

As indicated by the equation, the one equivalent weight of acid isliberated in the gel-forming reaction for each mole of magnesium saltused. If desired, additional HCl, beyond that liberated when bifluoridesare used, may be present, such as an amount up to about 250 per cent ofthe number of moles of magnesium salt so that the total number ofequivalent weights of acid involved may be as high as 350 per cent ofthe number of equivalent weights of the magnesium salt.

In preparing the gel, the solutions to be reacted, that is the solutioncontaining the mag nesium salt, the solution containing the fluoridesalt, and additional acid if used, are mixed together just before use.The amount of water used should be sufiicient to dissolve the solublemagnesium salt and fluoride salt used, the total amount in moles beingfrom about 12 to 121 times the number of equivalent weights of themagnesium salt. The mixing of the solutions may be carried out in anyconvenient manner as by pouring one solution into the other andsubjecting the resulting mixture to stirring until the two solutions areintimately mixed. In a few minutes, the mixture becomes viscous as aresult of the interaction of the magnesium ions with the fluoride ionsin the presence of the required acid. The viscous mixture changes in afew minutes of quiescent standing into a jelly-like viscous material,herein referred to as a gel, which owes its jelly-like consistency tothe presence, in the acidcontaining aqueous reaction mixture, ofhydrated magnesium fluoride. The jelly-dike properties of the gel arenot significantly changed on heating up to about 275 F. but onneutralizing the acid therein the jelly-like properties are destroyedand the gel thins to a consistency like that of the salt solutions fromwhich the gel is prepared. The gel may be caused to revert to thenon-gelled condition by subjecting the gel to agitation as by stirringthe gel or pumping it through a pipe in which agitation occurs duringflow. The property possessed by the gel of reverting to a non-gelledcondition on being subjected to agitation and of jelling again on beingheld quiescent, a property hereinbefore referred to as thixotropy, ishighly advantageous in its use in well treatment.

By virtue of the property of thixotropy, the agitated gel may beintroduced into the bore of a well through the usual tubing or casing asa thin fluid having a viscosity of not over about cps. and on leavingthe thin fluid in the well in a quiescent state it sets to a jelly-likemass which strongly resists displacement into the adjacent earth.Pressures high enough to lift the overburden can be attained in the wellhole containing the gel with attendant advantages of forming newchannels or cracks in the adjacent formation. The earth formationgenerally contains or comprises calcareous matter, such as calciumcarbonate and magnesium carbonate in various forms, e. g. limestone,dolomite, and these materials neutralize more or less of the acidin thegel while it is in the earth formation. As a result, the gel after awhile liquefies or thins in situ to a viscosity of 40 to 60 centipoisesand does not form a permanent block in the interstice of the earth.

As already mentioned, particulated solids, e. g. sand particles, whichin the aggregate form of a fluid permeable mass, may be carried by thegel into the channels and cracks on applying high pressure to the gel inthe well hole as above mentioned. When the gel liquefies, the particlespermeable mass, thereby facilitating the flow of, earth fluids to thewell through the formed cracks or channels. i

TABLE Gel Components in mole percent Viscosity of 1 Agitated gel M onemoNH FHF H2O Hon cenflpmses *Exclusive of that liberated in thegel-formation reaction.

It is manifest from the foregoing table that the gel, even in the fluidstate induced by agitation, possesses a high viscosity which renders iteffective to hold in suspension particulated solids, such as sand.

The following example is illustrative of various embodiments of theinvention:

Example A well 8000 feet deep is eased, the casing being perforated from7500 feet to 7525 feet. The formation opposite the perforations ismainly sandstone having a solubility of about 10 per cent by weight inhydrochloric acid. The temperature of the well at the bottom is 225 F. Astring of tubing carrying a packer is run into the well and the wellfilled with oil. The packer is located at 7490 feet and the lower end ofthe tubing at 7500 feet. In one tank, 3150 pounds of MgCl2.6HzO aredissolved in 247 gallons of water to make about 500 gallons of magnesiumchloride solution. In another tank, 900 pounds of NH4F.HF are dissolvedin 424 gallons of wa ter to make about 500: gallons of ammoniumbifluoride solution. At the well, the two tanks of solution are mixedtogether in a third tank for about 20 minutes and then 1000 pounds ofsand are mixed with the resulting gel. The resulting gel-sand mixture isthen pumped into the well tubing followed by oil. As the gel-sandmixture flows downwardly into the well, a corresponding volume of oil isdisplaced and allowed to flow from the casing. When the displaced oilvolume indicates that the gel-sand mixture has reached within 100 feetof the bottom of the tubing the packer is set. The injection of followeroil into the tubing string is continued while escape of fluid up thecasing is prevented by the packer. The pressure of the oil in the tubingincreases, and pressure is also maintained on the casing so as to assistthe packer in holding. The introduction of follower oil into the tubingis continued until the volume introduced indicates that the gel-sandmixture has reached the perforations. The introduction of follower oilthen is halted for 20 minutes to allow the gel-sand mixture to set to ajellied mass or become stiffened. After allowing the gell to stiffen,the introduction of follower oil is resumed under sufiicient pressure toforce the jelled gel-sand mixture through the perforations into theadjacent earth formation. The injection pressure required rises untilthe earth cracks or fractures allowing the gel-sand mixture to enter theearth. The introduction of follower oil into the tubing is continueduntil it is calculated that the entire volume of the gel-sand mixturehas been injected into the earth. After allowing the gel to becomethinned by contact with the earth formation, the pressure on the tubingis released and the well is put back on production.

Among the advantages of the invention are,

that the gel produces an effective barrier to the passage of fluid intoporous earth except under a pressure great enough to crack or fracturethe earth; the gel does not permanently plug earth pores as it liquefiesor thins in time as the alkaline constituents of the earth neutralizemore or less of the acid in the gel; the gel is thixotropic so that itis easily introduced into a well in its fluid state with conventionalequipment, yet stiifens or gels on quiescent standing, as in the wellhole or bore, when pore-plugging action is desired; the thixotropicproperty of the gel makes it hold solid particles, such as sand, insuspension with but slight, if any, tendency to settle even attemperatures as high as 400 F.

We claim:

1. The method of treating an earth formation ,penetrated by the bore ofa well which comprises introducing into the well an aqueous thixotropiccomposition comprising magnesium fluoride formed by reacting together anaqueous solution containing a water-soluble magnesium salt and anaqueous solution containing a water-soluble fluoride in the presence ofa strong mineral acid, the number of equivalent weights of the fluoridesalt being from to per cent of the number of equivalent weights of themagnesium salt and the number of equivalent weights of acid being from4.5 to 350 per cent of the number of equivalent weights of the magnesiumsalt, the number of moles of water in the magnesium salt solution plusthe number of moles of Water in the fluoride salt solution being betweenabout 12 and 121 .times the number of equivalent weights of themagnesium salt.

2. The method of treating an earth formation penetrated by a well borewhich comprises introducing into the well bore a composition comprisingan aqueous solution containing a watersoluble magnesium salt and anaqueous solution containing a water-soluble fluoride in the presence ofa strong mineral acid, the number of equivalent weights of the fluoridesalt being from 95 to 105 per cent of the number of equivalent weightsof the magnesium salt and the number of equivalent weights of acid beingfrom 45 to 350 per cent of the number of equivalent weights of themagnesium salt, the number of moles of Water in the magnesium saltsolution plus the number of moles of water in the fluoride salt solutionbeing between about 12 and 121 times the number of equivalent weights ofthe magnesium salt, said composition having a volume sufficient to fillthe well bore to a depth above that of the earth formation to betreated, allowing the said composition to remain in the well bore for a,time sufficient to induce jelling of the composition, and following thesaid composition with a fluid under pressure so as to force thecomposition after jelling into the earth formation.

3. The method according to claim 1 in which a particulated solid isadded to the thixotropic swam composition in: amount up to 4 pounds perthereof.

t. The method according to claim 2 which a particulated' solid is addedto the composition in amount up to 4 pounds per gallon thereof.

5. The method of treating an earth formation penetrated-F by the bore ofa we'll which comprises introducing into the well an aqueousthixotr'opic composition comprising magnesium fluoride tormed byreacting together an aqueous solution containing magnesium chloride andan aqueous solution containing ammonium bifiuoride", the number ofequivalent weights of the ammonium bifluoride being from 95 to 105 percent of the number of equivalents of the magnesium chloride, the numberof moles of water in the magnesium chloride solution plus the number ofmoles of water in the ammonium bifiuoride solution being between about12 and 121.

6'. A thi xotropic composition comprising the reaction product ofreacting together an aqueous solution of a water-soluble magnesium saltand} a water-soluble fluoride in the presence of a mineral acid thenumber of equivalent weights of the water-soluble fluoride being from 95to 105 per cent of the number of equivalent weights of the magnesiumsalt, and the number of equivalent wei'ghtsof the acid being from 45 to350 per cent of the number of equivalent weights of the magnesium salt,the number of moles of water in the magnesium salt solution plus thenumber of moles of water the fluoride solution. being between 12 and121.

'F. A- thi-xotropic composition according to claim it in which there isincluded per gallon thereof up to4 pounds of a particulated solid.-

8'. A thix'otropic composition according to claim 7 in which theparticulated solid is sand.

9. The method according to claim 1 followed by the application ofhydraulic pressure through the well bore suificient to force thecomposition into theearth formation and fracture it.

10', The method according to claim 2 followed by the application ofhydraulic pressure through the well bore sufiicient to force thecomposition into the earth formation and fracture it.

11. The method according to claim 5 followed by the application ofhydraulic pressure through the well bore sufii'ci'ent to force thecomposition into the earth formation and fracture it.

FRANK H. BRAUNLICH, JR. PAUL H. CARDWELL.

References Cited in the file of this patent UNITED STATES PATENTS NameDate 2,156,220 Dunn Apr. 25, 1939 2,236,147 Lerch et a1 Mar. 25, 19412,454,921 Heinemann Nov. 30, 1948 2,596,137 Fast 4. May 13, 19522,596,843 Farris a May 13, 1952 OTHER REFERENCES The Oil and. GasJournal, February 4, 1937, pp. 46, 48, 49 and 50.

1. THE METHOD OF TREATING AN EARTH FORMATION PENETRATED BY THE BORE OF AWALL WHICH COMPRISES INTRODUCING INTO THE WELL AND AQUEOUS THIXOTROPICCOMPOSITION COMPRISING MAGNESIUM FLUORIDE FORMED BY REACTING TOGETHER ANAQUEOUS SOLUTION CONTAINING A WATER-SOLUBLE MAGNESIUM SALT AND ANAQUEOUS SOLUTION CONTAINING A WATER-SOLUBLE FLUORIDE IN THE PRESENCE OFA STRONG MINERAL ACID THE NUMBER OF EQUIVALENT WEIGHTS OF THE FLUORIDESALT BEING FROM 95 TO 105 PER CENT OF THE NUMBER OF EQUIVALENT WEIGHTSOF THE MAGNESIUM SALT AND THE NUMBER OF EQUIVALENT WEIGHTS OF ACID BEINGFROM 45 TO 350 PER CENT THE NUMBER OF EQUIVALENT WEIGHT OF THE MAGNESIUMSALT, THE NUMBER OF MOLES OF WATER IN THE MAGNESIUM SALT SOLUTION PLUSTHE NUMBER OF MOLES OF WATER IN THE FLUORIDE SALT SOLUTION BEING BETWEENABOUT 12 AND 121 TIMES THE NUMBER OF EQUIVALENT WEIGHTS OF THE MAGNESIUMSALT.